This invention relates to a low-dispersion optical fiber used for example when wavelength division multiplexing optical transmission is carried out in the 1.5 xcexcm band, and to an optical transmission system using this low-dispersion optical fiber.
With the development of the information society the amount of information communicated has been increasing dramatically, and the realization of high bit-rate and high capacities in optical fiber communications has become an urgent and unavoidable issue. As an approach to this realization of more high bit-rate and capacities, optical fiber type optical amplifiers, which by using an optical fiber doped with a rare earth element, such as an erbium-doped optical fiber (EDF) doped with Er3+, can amplify an optical signal in the form of light, have been developed. And with the development the optical amplifiers which uses those optical fiber, the realization of high-power signal light has been progressing rapidly.
Meanwhile, to increase communication capacities in optical communications, communications using wavelength division multiplexing optical transmission, wherein optical signals having different wavelengths are transmitted down a single optical fiber, have been being developed. And from the application of the optical amplifier which uses above-mentioned optical fiber to optical communication using this wavelength division multiplexing optical transmission (wavelength multiplexing optical transmission systems), further increases in communication capacity and the realization of long-distance transmission are anticipated.
One representative example of an optical fiber type optical amplifier is the EDFA (Erbium-Doped optical Fiber Amplifier), which has an EDF of the kind mentioned above. The use of the EDFA to conduct the above-mentioned wavelength division multiplexing optical transmission with the 1.5 xcexcm wavelength band (wavelength 1520 nm to 1620 nm), which is the gain band of the EDFA, as the transmission band has been being studied.
FIGS. 6A and 6B show examples of refractive index profiles of optical fibers that have been proposed in related art as optical fibers for wavelength division multiplexing optical transmission with, of the above-mentioned 1.5 xcexcm wavelength band, particularly the 1550 nm vicinity wavelength band (the 1.55 xcexcm wavelength band) as the transmission band (used wavelength band). FIG. 6A shows a dual shaped refractive index profile, and FIG. 6B shows a W-shaped refractive index profile.
The optical fiber with the dual shaped refractive index profile is made up of a cladding 5, a center core 1 having a larger refractive index than that of the cladding 5, and a first side core 2 having a refractive index smaller than that of the center core 1 but larger than that of the cladding 5. The optical fiber with the W-shaped refractive index profile is made up of a cladding 5, a center core 1 having a larger refractive index than that of the cladding 5, and a first side core 2 having a refractive index smaller than that of the cladding 5.
Among optical fibers with the dual shaped refractive index profile described above, those having their zero dispersion wavelength in the 1.55 xcexcm wavelength vicinity are called dispersion-shifted optical fibers. Because a dispersion-shifted optical fiber has its zero dispersion wavelength in the vicinity of the wavelength 1.55 xcexcm, which is the center wavelength of the 1.55 xcexcm wavelength band, distortion of the signal light waveform caused by dispersion in the 1.55 xcexcm wavelength band is suppressed. On the down side, however, the occurrence of the nonlinear phenomenon of four-wave mixing is marked. Consequently, with this dispersion-shifted optical fiber, four-wave mixing light arising causes distortion to occur in the waveform of the signal light, and it is impossible to realize high-quality wavelength division multiplexing optical transmission.
To overcome this, dual shaped refractive index profile optical fibers having their zero dispersion wavelength shifted from the 1.55 xcexcm wavelength band have been developed. However, it is known that in this kind of optical fiber the dispersion gradient in the 1.55 xcexcm wavelength band is large. And because of that, with this kind of optical fiber it is difficult to make small the chromatic dispersion differential in the used wavelength band in wavelength division multiplexing optical transmission (the difference between the maximum value and the minimum value of the chromatic dispersion in the used wavelength band). Consequently, when this kind of optical fiber is used, it is not possible for the used wavelength band used for wavelength division multiplexing optical transmission to be made wide.
An optical fiber having the W-shaped refractive index profile functions as a dispersion flattened optical fiber, because the above-mentioned chromatic dispersion differential is small. However, whereas the effective core area (the region through which light effectively propagates: Aeff) of the dual shaped refractive index profile optical fiber is about 45 xcexcm2, the effective core area of a W-shaped refractive index profile optical fiber is for example about 30 xcexcm2, or about ⅔ of that of the dual shaped refractive index profile optical fiber. And when the effective core area is small like this, in wavelength division multiplexing optical transmission there has been the problem that the transmitted signal deteriorates as a result of nonlinear phenomena arising in the optical fiber.
To overcome this, the idea of increasing the effective core area by using an optical fiber having a segment core refractive index profile of the kind shown in FIG. 6C has been proposed. In FIG. 6C, 1 denotes a center core; 2 a first side core; 3 a second side core; and 5 a cladding. However, with this kind of optical fiber, because the chromatic dispersion gradient in the 1.5 xcexcm wavelength band is large and the chromatic dispersion differential in the same wavelength band is large, when the optical fiber of this proposal is applied to wavelength division multiplexing transmission, the problem arises that signal light waveform deterioration caused by chromatic dispersion becomes marked.
Also, to apply an optical fiber to a wavelength division multiplexing transmission system, the optical fiber must be incorporated into a cable. And because the cable is required to have the property that loss increases caused by bending of the optical fiber and side pressures on the optical fiber are low, it is also required of an optical fiber for wavelength division multiplexing transmission use that its bending property be good.
However, as explained above, there has not yet been realized an optical fiber with which it is possible to obtain both the effective core area and the reduced chromatic dispersion differential necessary to realize a high-quality wavelength division multiplexing transmission system, and additionally it has been difficult to realize an optical fiber whose bending loss property are also good.
Also, in recent years, as optical amplifiers, the Raman amplifier has been approaching practical introduction. The Raman amplifier has a wider amplifiable wavelength band than existing EDFAs, and can amplify a light signal of any specified wavelength band within for example the wavelength range of 1450 nm to 1650 nm. However, studies of optical fibers in this wavelength range have not yet advanced.
It is therefore an object of the present invention to provide a low-dispersion optical fiber with which it is possible to obtain both increased effective core area and reduced chromatic dispersion differential in a used wavelength band and furthermore to reduce loss increases caused by bending and side pressures when the optical fiber is made into a cable, and an optical transmission system using this low-dispersion optical fiber.
A low-dispersion optical fiber of a first construction provided by the invention to achieve this and other objects is a dispersion-shifted optical fiber made by covering a center core with a first side core, covering the first side core with a second side core, and covering the second side core with a cladding, characterized in that when the maximum refractive index of the center core is written n1, the minimum refractive index of the first side core is written n2, the maximum refractive index of the second side core is written n3 and the refractive index of the cladding is written nc, then n1 greater than n3 greater than nc greater than n2; the relative refractive index difference xcex941 with respect to the cladding of the maximum refractive index of the center core is 0.4%xe2x89xa6xcex941xe2x89xa60.7%; the relative refractive index difference xcex942 with respect to the cladding of the minimum refractive index of the first side core is xe2x88x920.30%xe2x89xa6xcex942xe2x89xa6xe2x88x920.05%; the relative refractive index difference xcex943 with respect to the cladding of the maximum refractive index of the second side core is 0.2% less than xcex943; the ratio (a1/a2) of the diameter a1 of the center core to the diameter a2 of the first side core is at least 0.4 and not greater than 0.7; and the ratio (a3/a2) of the diameter a3 of the second side core to the diameter a2 of the first side core is not greater than 1.6.
A low-dispersion optical fiber of a second construction provided by the invention is characterized in that, in addition to the first construction described above, the second side core is doped with an additive which raises the refractive index of SiO2; the concentration distribution in the optical fiber radial direction of the additive doped into the second side core has a peak; and the position of the peak is on the first side core side of the radial direction center of the second side core.
A low-dispersion optical fiber of a third construction provided by the invention is characterized in that, in addition to the second construction described above, the additive is GeO2.
A low-dispersion optical fiber of a fourth construction provided by the invention is characterized in that, in addition to the first or the second or the third construction described above, a low refractive index cladding part of a smaller refractive index than the cladding is provided between the cladding and the second side core.
A low-dispersion optical fiber of a fifth construction provided by the invention is characterized in that, in addition to the first or the second or the third construction described above, it does not have zero dispersion wavelength in a used wavelength band included in the 1450 nm to 1650 nm wavelength band.
A low-dispersion optical fiber of a sixth construction provided by the invention is characterized in that, in addition to the fourth construction described above, it does not have zero dispersion wavelength in a used wavelength band included in the 1450 nm to 1650 nm wavelength band.
A low-dispersion optical fiber of a seventh construction provided by the invention is characterized in that, in addition to the first or the second or the third or the sixth construction described above, the differential between the maximum value and the minimum value of the dispersion value in a wavelength band having an arbitrary bandwidth of 30 nm included in the wavelength band 1450 nm to 1650 nm is not greater than 2 ps/nm/km.
A low-dispersion optical fiber of an eighth construction provided by the invention is characterized in that, in addition to the fourth construction described above, the differential between the maximum value and the minimum value of the dispersion value in a wavelength band having an arbitrary bandwidth of 30 nm included in the wavelength band 1450 nm to 1650 nm is not greater than 2 ps/nm/km.
A low-dispersion optical fiber of a ninth construction provided by the invention is characterized in that, in addition to the fifth construction described above, the differential between the maximum value and the minimum value of the dispersion value in a wavelength band having a bandwidth of 30 nm included in the wavelength band 1450 nm to 1650 nm is not greater than 2 ps/nm/km.
An optical transmission system of a tenth construction provided by the invention is characterized in that it has an optical transmission line including a low-dispersion optical fiber of any one of the first through ninth constructions described above and a dispersion-compensating device whose chromatic dispersion gradient in the wavelength band 1450 nm to 1650 nm is negative, and a positive chromatic dispersion gradient of the optical transmission line in this wavelength band is reduced by the dispersion-compensating device.
In this specification, the specific refractive indexes xcex941, xcex942 and xcex943 mentioned above are defined by the following expressions (1) through (3).
xcex941={(n12xe2x88x92nc2)/2nc2}xc3x97100xe2x80x83xe2x80x83(1) 
xcex942={(n22xe2x88x92nc2)/2nc2}xc3x97100xe2x80x83xe2x80x83(2) 
xcex943={(n32xe2x88x92nc2)/2nc2}xc3x97100xe2x80x83xe2x80x83(3) 
A low-dispersion optical fiber according to the invention has a first object of providing in a set wavelength band for example within the wavelength range of 1450 nm to 1650 nm both an increased effective core area and a reduced chromatic dispersion differential in the used wavelength band. A low-dispersion optical fiber according to the invention has its refractive index distribution and its core diameter ratios optimized so that it is possible to achieve this first object and also to reduce loss increases caused by bending and side pressures when the optical fiber is made into a cable. Thus, with a low-dispersion optical fiber according to the invention, it is possible to obtain both an increased effective core area and a reduced chromatic dispersion differential in the used wavelength band and furthermore to reduce loss increases caused by bending and side pressures when the optical fiber is made into a cable. Specific examples of low-dispersion optical fibers according to the invention will be discussed hereinafter in the section on modes of practicing the invention.
In one construction of a low-dispersion optical fiber according to the invention, the second side core is doped with an additive which raises the refractive index of SiO2; the concentration distribution in the optical fiber radial direction of the additive doped into the second side core has a peak; and the position of the peak is on the first side core side of the radial direction center of the second side core. And in another construction, a low refractive index cladding part of a smaller refractive index than the cladding is provided between the cladding and the second side core.
In these constructions, effective cutoff wavelength can be made short. Consequently, with these constructions, it is possible to achieve a still greater increase in effective core area and a still greater reduction in the chromatic dispersion differential in the used wavelength band, and a superior low-dispersion optical fiber capable of single mode operation can be obtained.
Also, in a construction wherein, as described above, a peak in the concentration distribution in the optical fiber radial direction of an additive doped into the second side core which raises the refractive index of SiO2 is positioned on the first side core side of the radial direction center of the second side core, if the additive is made GeO2, the optical fiber can be made easily using existing optical fiber manufacturing technology.
And if a low-dispersion optical fiber according to the invention is given a construction such that it does not have zero dispersion wavelength in a used wavelength band within the wavelength range of 1450 nm to 1650 nm, for example in the wavelength band 1530 nm to 1560 nm, if for example wavelength division multiplexing optical transmission is carried out in this wavelength band, the occurrence of four-wave mixing can be suppressed and waveform distortions caused by nonlinear phenomena can consequently be suppressed. The above-mentioned used wavelength band can be set freely within the wavelength range of 1450 nm to 1650 nm.
And if in a low-dispersion optical fiber according to the invention the differential between the maximum value and the minimum value of the dispersion value in the above-mentioned wavelength band is made 2 ps/nm/km or below, when for example wavelength division multiplexing optical transmission is carried out in this wavelength band, waveform distortions caused by chromatic dispersion can be certainly suppressed.
An optical transmission system according to the invention uses an optical transmission line including a low-dispersion optical fiber described above and furthermore the positive chromatic dispersion gradient in the wavelength band 1450 nm to 1650 nm of this optical transmission line including a low-dispersion optical fiber is reduced by means of a negative chromatic dispersion gradient of a dispersion-compensating device. With an optical transmission system according the invention, because the chromatic dispersion gradient in the above-mentioned wavelength band can be made to approach zero and the influence of chromatic dispersion can be suppressed still more, it is possible to build an optical transmission system capable of high-quality wavelength division multiplexing transmission.